Fuels which are gaseous at normal temperature and pressure, such as hydrogen methane, ethane, propane, butane, and carbon monoxide, have been used as fuels for motor vehicles for at least a century. The use of such fuels has always been limited to a very small fraction of the world's motor vehicles.
Liquid fuels, that is fuels that are liquid at normal temperatures and pressures, have been the dominant motor vehicle fuels by far.
Geological estimates of the availability of fossil fuels in the earth's crust, indicate that approximately equal quantities, on an energy equivalent basis, exist for both liquid and gaseous fuels. The gaseous fuels in nature are dominantly methane. Typically over 90 percent of natural gas is composed of methane.
The domination of liquid fuels for motor vehicle use in primarily because of their high energy density on a volume basis when compared to gaseous fuels. This permits an extended driving range between refueling intervals for liquid fuels.
Gaseous fuels, especially methane and hydrogen, generally have a higher ratio of hydrogen to carbon in the molecule and hence a higher combustion energy per unit weight than liquid fuels. Methane has 116%, and hydrogen has 279% of the energy per unit weight than gasoline.
At normal temperature and pressure, the volumetric energy density of methane is about one thousandth and hydrogen is about one twenty-five hundredth that of liquid fuels.
To provide a reasonable driving range between refueling intervals, prior art storage systems have increased the energy density of gaseous fuels by several methods. In one, the gaseous fuel is cooled to a sufficiently low temperature so that the gas liquefies. When the gaseous fuel is methane, that temperature is at most -165.degree. C. at normal pressure. This is called cryogenic storage. The volumetric energy density is then about 89% that of gasoline. An even lower temperature of -254.degree. C. is required to liquefy hydrogen. The volumetric energy density of hydrogen, when so liquefied, is about 28% that of gasoline.
At such a low temperature, the fuel must be stored in a special, very well insulated, cryogenic dewar if it is to remain in liquid form for a reasonable time. The equipment required to liquefy, store, and then regasify such fuels is very sophisticated and requires special care on the part of those who operate such systems. For safety, it is necessary to avoid contact with the cryogenic temperatures, and also to ensure that when fuel storage systems warm up suitable precautions are taken to avoid high pressure buildup in the storage vessel. These characteristics have made cryogenic storage of gaseous fuels a rarely used gaseous fuel storage system for motor vehicles.
The second and more widely used storage system has been compression of the gaseous fuel. In order to minimize the weight of storage vessels, as compared to the weight of gaseous fuel stored therein, a pressure of approximately 200 times normal atmospheric pressure is widely employed. These systems are called compressed gas systems. When methane is the gaseous fuel, the energy density on a volumetric basis is approximately one fourth that of gasoline, the most widely used liquid fuel. Therefore, a compressed natural gas (CNG) storage system provides either approximately one fourth of the driving range of a gasoline fueled engine for the same fuel storage volume or, conversely, approximately four times the storage volume required for the same range.
One consequence of this low energy density is that most vehicles operating on gaseous fuels are usually provided with a bi-fuel capability. That is, equipment is provided to permit the engine to operate on a conventional liquid fuel such as gasoline, as well as on the gaseous fuel. This requires separate storage tanks, carburetors, and all the other necessary equipment to operate on two completely different fuels. Such operating capability is called "bi-fuel capability".
Fuels that have a critical temperature below normal temperature (20.degree. C.) [which] I will call "low critical temperature fuels" (LCT fuels) [will not be liquid at any pressure above their critical temperatures]. Examples of such fuels are methane and hydrogen. Fuels that have a critical temperature above normal temperature, I will call "high critical temperature fuels" (HCT fuels). They can be liquified at normal temperature. Examples of such fuels are ethane, propane, butane, methanol, ethanol and gasoline.
Above its critical temperature a fuel cannot be liquified at any pressure.
Because HCT fuels can be stored as liquids at modest pressures, they can achieve moderately high energy densities (megajoules/meter.sup.3), and can therefore provide a reasonable driving range for a vehicle in which they are employed. Unfortunately, the availability of HCT fuels, which can also be gaseous at normal temperatures, is limited. Ethane, propane and butane represent only a small fraction of the content of natural gases recovered from the earth. These fuels are also produced as small byproducts of refinery operations. Therefore, use of such fuels as important transportation fuels must be limited.
What is provided by the present invention is a method for eliminating the need for bi-fuel capability in vehicle combustion engines when it is intended to have such combustion engines operate primarily on LCT fuels in motor vehicles, by increasing the range of the vehicle, per unit volume of storage space, by more than 2 times, thereby halving the number or volume of storage tanks required for a given range, or conversely, doubling the range between fuel refills by at least a factor of two.
The present invention can reduce or eliminate the need for bi-fuel capability to increase driving range, thus eliminating the expense and weight of a bi-fuel system, while retaining the environmentally benign advantages of gaseous fuels over the total driving cycle of the vehicle.